The Cellular Organelles and Function Page! (cell diagram)
Cells are usually 8-20 mm in diameter. The resolving power of the human eye is roughly 100 mm, so we cannot see cells with the "naked eye." The resolving
power of a light microscope is around 0.2 mm, so we can see cells fairly easily if they are adequately stained. The resolving power of the electron microscope is 0.2 nm (where 0.1 nm or 1 ten-billionth of a meter = 1
Angstrom (Å)), so it is possible to perform a detailed examination of cellular organelles with an electron microscope.
PROTOPLASM is a word used to describe the substances of which cells are composed.
Cell Membrane (Plasmalemma) and Associated Proteins
The Singer-Nicholson "fluid mosaic model" of the cell membrane
The cell membrane separates the cell contents (organelles) from the environment. Singer and Nicholson described the cell membrane as a phospholipid bilayer. The phosphates are hydrophilic (polar) and project outward while the lipid groups are hydrophobic (nonpolar) and point inwards. Membranes are selective against substances which are not lipid soluble, substances which are polar and substances which are too big. It should be noted, however, that very small polar molecules (like H2O) can pass at limited rates through membranes.
Membrane proteins (intrinsic, extrinsic, trans-membrane or membrane-spanning) are capable of random or directed lateral movement (they float) through the membrane.
functional classes of membrane proteins
i) transport proteins (docking proteins, ionophores or ion channels, ion conductance or carrier proteins, energy driven ion pumps)
ii) receptor proteins recognize specific extracellular messages from the "biological soup" and activate intracellular signal transduction mechanisms (enzymes) to cause biological responses
iii) membrane-associated enzymes (often associated with hormone receptors)
iv) junctional proteins and cytoskeletal anchorage points
-cell-cell adhesion or cell-cell communication
-desmosomes (cell to cell anchorage proteins)
-gap junctions (nexus); 6 wedge-shaped connexons/doughnut
-tight junctions; seams? - tight and leaky epithelial/ endothelial cells
-juxtacrinology; membrane bound hormones may interact with membrane bound receptors on other cells (such relationships may exist between circulating cells)
v) recognition sites (eg. antigenic epitopes)
-proteins have 3-dimensional projections with unique characteristics that allow lymphocytes to develop antibodies against proteins
Cytoplasm (Cytosol) and the Cytoskeleton (cell diagram)
-cytosol is intracellular fluid
-the cytoskeleton = a variety of intra- and inter-cellular connecting proteins
-there are 3 major protein strand types in the cytoskeleton
i) microfilaments; smallest, actin; movement, transport
ii) intermediate filaments; a variety of proteins; the most stable cytoskeletal proteins; involved in structural integrity
iii) microtubules; largest, hollow tubulin; involved in movement, transport
The Organelles (cell diagram)
Endoplasmic Reticulum (ER)
-involved in synthesis of proteins (the rough ER = RER), phospholipids, fatty acids and steroids (the smooth ER = SER)
-involved in packaging of products destined for transport within or out of cells
-the major intracellular Ca2+ storage site (especially the smooth (SER) and the sarcoplasmic reticulum in muscle)
Golgi Complex (Golgi Apparatus; Golgi Bodies) -a specialized portion of the ER involved in packaging of products destined for transport within or out of cells
-involved in post-translational modification of proteins
-involved in production of lysosomes and packaging of enzymes into lysosomes for lysosomal destruction of proteins, sugars, nucleotides, amino acids ("recycling" - ie. break down old stuff to provide more substrate to make new stuff)
Lysosomes (hydrolytic) & Peroxisomes (oxidative)
-"enzyme-loaded" vesicles; the cell's "internal digestive system"
-lysosomes are formed by "budding off" of the Golgi complex
-peroxisomes are formed by "budding off" of the smooth endoplasmic reticulum
-destroy materials engulfed by phagocytosis or endocytosis and destroy damaged organelles ("autophagy")
Thus, the endoplasmic reticulum, Golgi apparatus and lysosomes work in harmony to synthesize and recycle cellular materials.
Vacuoles
-a space in the cytoplasm filled with H2O and solutes, within a single membrane
Centrioles (part of the cytoskeleton)
-small protein cylinders which act as anchorage points for spindle fibers which pull apart paired chromosomes during cell
division.
Secretory Vesicles (Secretory Granules)
-vesicles produced by the Golgi complex containing proteins synthesized by the rough endoplasmic reticulum
-the contents of the vesicles are destined for incorporation into the cell membrane or secretion from the cell
Nucleus and Nucleolus
-the nucleus is the largest organelle; it has a porous membrane allowing RNA but not DNA passage (mRNA codes for protein synthesis by ribosomes); it contains chromosomes; mature mammalian RBC's are anucleate (ie. they have no nucleus)
-the nucleus contains the hereditary information in the form of DNA which codes for the synthesis of proteins
-the nucleus does not really regulate cellular activity; rather, factors entering the nucleus (steroids, enzymes, etc) regulate mRNA expression and protein synthesis
-the nucleolus is an association of DNA and protein; it is the site of ribosomal RNA and basic ribosome assembly; no membrane surrounds the nucleolus
Mitochondria & Energy Metabolism
Mitochondria have a double bilayer membrane with extensive folding of inner membrane. The folds of the inner membrane are called cristae and they increase the surface area of the inner membrane. This is important as the inner membrane
is one of the sites where the most powerful ATP-producing process occurs (the electron transport chain). The mitochondria carry out oxidative metabolism of CHO's (primarily glucose) and fatty acids to capture energy in the form of ATP while
releasing CO2 and H2O as waste. Oxidative means that the mitochondria work only in the presence of oxygen!
